1. Field of Invention
This invention relates to the field of storing and transferring spent nuclear fuel and specifically to a method and apparatus for ensuring that casks used to store and transfer spent nuclear fuel do not tip over during seismic events.
2. State of the Art
In the operation of nuclear reactors, it is customary to remove fuel assemblies after their energy has been depleted down to a predetermined level. Upon removal, this spent nuclear fuel is still highly radioactive and produces considerable heat, requiring that great care be taken in its packaging, transporting, and storing. In order to protect the environment from radiation exposure, spent nuclear fuel is both transported and stored in large cylindrical containers called casks. A transfer cask is used to transport spent nuclear fuel from location to location while a dry storage cask is used to store spent nuclear fuel for a determined period of time.
In performing their respective functions, there are several steps in which a dry storage cask and a transfer cask must be arrayed in a vertical orientation. Steps such as welding a multi-purpose canister (MPC) lid, removing water from the MPC, and backfilling the MPC with helium, require several hours during which plant personnel must work in close proximity to a transfer cask while it is in a vertically oriented position. A transfer cask has a sizable footprint (about 8 feet) in relation to its height (about 16 feet), and is therefore quite adequate to stand in a freestanding configuration to perform the required operations at most plants. However, at certain nuclear plants the postulated design basis earthquake (DBE), particularly in areas of high seismic activity, creates the danger that a vertically oriented cask can tip over during a seismic event. This is a matter of credible safety concern.
As a result of this danger, nuclear plants are required to erect a temporary restraint system if a cask is found to tip over in analytical simulation of the effects of the postulated design basis earthquake. Because of the great mass and bulk of the structures involved (e.g. a loaded cask can typically weigh nearly 250,000 lbs), the restraint system is a bulky structure as well. As such, building and installing such large structures to protect the cask from tipping over during loading operations is very costly. Furthermore, because all of the loading steps cannot occur at one location, more than one temporary restraint system is typically required to protect a cask from tipping over if an earthquake were to occur during the fuel loading operations.
In addition to cost, installing the restraint systems inside the plants entails radiation dose to personnel during their erection, and even more so during fuel loading because of the reduction in access introduced by the presence of the restraint structure. It is estimated that the time that plant personnel must spend loading and “prepping” an MPC is at least doubled due to the presence of the restraint system.